We propose basic research into the use of reactive amendments as an alternative remediation technology for hazardous metal and metalloid contaminants (namely, As, Hg, Se, Pb, Cu) of high priority in sediments at Superfund and other contaminated sites. These inorganic contaminants are particularly problematic because they do not degrade, they pose chronic health hazards to humans and organisms at low concentrations, they can be widely dispersed, they may be remobilized by changing environmental conditions, and many can bioaccumulate. Our approach is aimed at developing a molecular-chemical understanding of element sequestration mechanisms as a result of in situ reaction between the amendment and contaminated sediments. Using laboratory experiments with sediments from contaminated sites, we will nvestigate contaminant sequestration reactions and aging of the amendment products, with the aim of optimizing novel treatments to maintain or increase their stability as sedimentary environments change from oxidized to reduced. We will employ advanced spectroscopic and microscopic characterizations, including synchrotron X-ray methods, to determine contaminant element speciation, mechanism(s) of sequestration, and host phases at the molecular-to-microscopic scale in complex sediment mixtures. Our proposed work will also evaluate the feasibility of adapting existing engineering technologies for the delivery of reactive amendments to field systems. Because few remediation alternatives exist presently for contaminated sediments, this research will fill gaps in basic knowledge about the long-term fate of sediment amendments and their sequestered contaminants in subsurface environments, which presently contributes to a lack of acceptance and use of this approach. New methods for the sequestration and immobilization of inorganic contaminants in the environment will have positive impacts on public health by reducing their ability to enter the food chain and thus their potential bioaccumulation and biomagnification.